This invention relates to the recovery of marketable products such as oil and gas from hydrocarbon bearing deposits such as oil shale or tar sand by the application of radio frequency energy to heat the deposits. More specifically, the invention relates to an arrangement of conductors, inserted in the formation, for applying the energy to achieve approximately uniform, elevated temperatures in a selected volume of material in the formation.
This country's reserves of oil shale and tar sand contain enough hydrocarbonaceous material to supply this nation's liquid fuel needs for many years. A number of proposals have been made for processing and recovering hydrocarbonaceous deposits, which are broadly classed as "in situ" methods. Such methods may involve underground heating or retorting of material in place, with little or no mining or disposal of solid material in the formation. Useful constituents of the formation, including heated liquids of reduced viscosity, may be drawn to the surface by a pumping system or forced to the surface by injection techniques. It is critical to the success of such methods that the amount of energy required to effect the extraction be minimized. Unfortunately, exploitation of hydrocarbonaceous deposits employing conventional in situ technology has not occurred on a large scale for economic reasons.
It has been proposed that relatively large volumes of hydrocarbonaceous formations be heated in situ using radio frequency energy. These proposals are exemplified by the disclosures of the following patents: U.S. Pat. No. 4,144,935 to Bridges et al, now U.S. reissue application Ser. No. Re. 118,957 filed Feb. 2, 1980 now U.S. Pat. No. Re. 30,738; U.S. Pat. No. 4,140,180 to Bridges et al, U.S. Pat. No. 4,135,579 to Rowland et al; U.S. Pat. No. 4,140,179 to Kasevich et al; and U.S. Pat. No. 4,193,451 to Dauphine.
The attainment of controlled or uniform temperature heating of a volume to be recovered is a desirable result. Non-uniform temperature distributions can result in the necessity of inefficient overheating of portions of the formations in order to obtain the minimum average heating necessary to facilitate recovery of the useful constituents in the bulk of the volume being processed. Extreme temperatures in localized areas may cause damage to the producing volume such as carbonization and arcing between the conductors.
Dauphine et al teaches techniques for attaining a more uniform dispersion of a radio frequency field. Rowland likewise indicates a preference for a uniform field pattern in discussing his four conductor embodiments shown in his FIG. 3. Finally, the Bridges et al disclosures teach the desirability of achieving uniform heating of a particular volume of the hydrocarbonaceous material. Embodiments disclosed by Bridges et al call for the heating of blocks of oil shale or tar sand by enclosing or bounding of the volume in an electrical sense with arrays of spaced conductors. One such array consists of three spaced rows of conductors which form the so-called "triplate-type" of transmission line structure similar to that shown in FIG. 2 of this application.
Uniformity of heating is predicted by Bridges et al as a result of a time-averaged uniformity in the intensity of the electric field within the triplate structure. This approximation assumes that the diminution of the electric field in any direction due to transfer of energy to the formations is not so severe as to cause undue non-uniformity of heating in the volume and wasteful overheating of portions thereof.
Despite the application of uniform fields, which are predicted to cause uniform heating, non-uniformity of temperature has been observed in tests employing the Bridges triplate structure. This non-uniformity may be caused by heat loss to the formation surrounding the bounded volume. As a result, in at least some formations and configurations of the bounded volume, the extremities of the volume may be significantly cooler than the central portion of the volume.
Accordingly, it is a feature of the present invention that subsurface formations be heated to a controlled or uniform temperature with radio frequency energy.
It is another feature of the present invention that a volume of hydrocarbonaceous material heated with radio frequency energy be configured to minimize heat loss at the extremities of the volume.
It is another object of the present invention to provide an apparatus and method for heating a volume of hydrocarbonaceous material to uniform temperatures in situ by compensating for heat loss to the surrounding formation.
The substantial confinement of the radio frequency energy to the volume of material which is to be heated is important for feasible extraction techniques. This is so for two reasons. First, the application of radio frequency energy to surrounding material which are not heated sufficiently to permit production of oil and gas is a waste of that energy. Second, large amounts of radiated radio frequency energy may interfere with radio cummunications above-ground.
Accordingly, it is another feature of the present invention that a subsurface volume in an earth formation be heated in a controlled or uniform fashion with radio frequency energy, while minimizing radiation of the radio frequency energy into surrounding environs.
These and other features of the invention will become apparent from the claims, and from the following description when read in conjunction with the accompanying drawings.